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LM2751 データシート(PDF) 9 Page - National Semiconductor (TI) |
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LM2751 データシート(HTML) 9 Page - National Semiconductor (TI) |
9 / 11 page Application Information CIRCUIT DESCRIPTION The LM2751 is a Switched Capacitor Convertor with gains of 2x and 1.5x. It is capable of continuously supplying up to 150mA at 4.5V or up to 80mA at 5V depending on the output voltage option. The LM2751’s fixed frequency pre-regulation maintains the output voltage to within 3% (typ.), making it well suited for driving White LEDs. There are also four user programmable switching frequencies to reduce the quies- cent current consumption at light loads. Aside from powering LEDs, the LM2751 is suitable for driv- ing other devices with power requirements up to 150mA. The LM2751 operates over the extended Li-Ion battery range from 2.8V to 5.5V. The LM2751 limits output current to 250mA (typ.) during an output short circuit condition. LED brightness is controlled by applying a PWM (Pulse Width Modulation) signal to the Enable pin (EN). (see PWM BRIGHTNESS CONTROL section). SOFT START Soft Start is engaged when the device is taken out of Shut- down mode (EN = logic HIGH) or when voltage is supplied simultaneously to the V IN and EN pins. During Soft Start, the voltage on V OUT will ramp up in proportion to the rate that the reference voltage is being ramped up. The output voltage is programmed to rise from 0V to the regulated output voltage level (4.5V or 5V) in 300µs (typ.). ENABLE MODE The Enable logic pin (EN) disables the part and reduces the quiescent current to 0.77µA (typ.). The LM2751 has an active-high enable pin (LOW = shut down, HIGH = operat- ing) which can be driven with a low-voltage CMOS logic signal (1.5V logic, 1.8V logic, etc). There is an internal 1M Ω pull-down resistor between the EN and GND pins of the LM2751. FREQUENCY MODE SELECT The LM2751 switching frequency is user programmable via two logic input pins, CS0 and CS1. Both logic input pins have active-high logic (LOW = un-selected, HIGH = selected) and can be driven with a low-voltage CMOS logic signal (1.5V logic, 1.8V logic, etc). There are no internal pull-down or pull-up resistors between the CSx and GND pins of the LM2751. The CSO and CS1 can be controlled independently or with the same logic signal. The selectable switching frequencies are 9.5kHz, 37kHz, 300kHz, 725kHz. The switching frequency is programmed according to Table 1 TABLE 1. Frequency Modes CS0 CS1 Frequency 0 0 37kHz 0 1 300kHz 1 0 9.5kHz 1 1 725kHz V OUT REGULATION The LM2751 uses pre-regulation to regulate the output volt- age to 4.5V or 5.0V depending on the voltage option. Pre- regulation uses the voltage present at V OUT to limit the gate drive of the switched capacitor charge pump. This regulation is done before the voltage is gained up by the charge pump, giving rise to the term "pre-regulation". Pre-regulation helps to reduce input current noise and large input current spikes normally associated with switched capacitor charge pumps. The LM2751 switched capacitor charge pump has gains of 2x and 1.5x. When the input voltage to the device is greater than 3.58V (typ.), the LM2751 operates in a gain of 1.5x. When the input voltage falls below 3.5V (typ.), the device switches to a gain of 2x. OUTPUT VOLTAGE RIPPLE The primary contributor in keeping the output voltage ripple of the LM2751 low is its switching topology. The output capacitance, input voltage, switching frequency and output current also play a significant part in determining the output voltage ripple. Due to the complexity of the LM2751 opera- tion, providing equations or models to approximate the mag- nitude of the ripple cannot be easily accomplished. However, the following general statements can be made. The LM2751 has very low output ripple when compared to typical boost regulators due to its double-pump topology, where charge is continually supplied to the output during both 2x and 1.5x modes. Combined with fixed frequency operation modes, double-pumping allows for the use of a very small, low value ceramic capacitor on the output node while still achieving minimal output ripple. Increasing the capacitance by adding a higher value capacitor or placing multiple capacitors in parallel can further reduce the ripple magnitude. CAPACITOR SELECTION The LM2751 requires 4 external capacitors for proper opera- tion. Surface-mount multi-layer ceramic capacitors are rec- ommended. These capacitors are small, inexpensive and have very low equivalent series resistance (ESR, ≤15mΩ typ.). Tantalum capacitors, OS-CON capacitors, and alumi- num electrolytic capacitors are generally not recommended for use with the LM2751 due to their high ESR, as compared to ceramic capacitors. For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with the LM2751. These capacitors have tight capacitance tolerance (as good as ±10%), hold their value over temperature (X7R: ±15% over −55˚C to 125˚C; X5R: ±15% over −55˚C to 85˚C), and typically have little voltage coefficient when com- pared to other types of capacitors. However selecting a capacitor with a voltage rating much higher than the voltage it will be subjected to, will ensure that the capacitance will stay closer to the capacitor’s nominal value. Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the LM2751. Capacitors with these temperature characteristics typically have wide ca- pacitance tolerance (+80%, −20%), vary significantly over temperature (Y5V: +22%, −82% over −30˚C to +85˚C range; Z5U: +22%, −56% over +10˚C to +85˚C range), and have poor voltage coefficients. Under some conditions, a nominal 1µF Y5V or Z5U capacitor could have a capacitance of only 0.1µF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum capacitance requirements of the LM2751. www.national.com 9 |
同様の部品番号 - LM2751 |
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同様の説明 - LM2751 |
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